The field of the invention pertains to electric servo motor drives and controllers and, in particular, to alternating current (AC) servo motor drives.
In general the speed of an electric motor is proportional to the voltage applied across the motor windings and the torque is proportional to the current flowing through the motor windings. For large motors where current demand is beyond that available from a single drive, the drives must be successfully connected in parallel. Thus, the drives must be designed to each supply a portion of the required current at whatever voltage is necessary to cause the required current to flow.
Methods for paralleling drives for brushed direct current (DC) motors are well known in the industry. Products exist and are available from a number of vendors which allow DC drives to be connected in parallel to drive one brushed DC motor. However, these methods and products control only the magnitude of the current delivered from each of the parallel drives. Such drives are relatively simple because the phase of the voltage and phase of the current (the output current vector) can be ignored.
In order to successfully parallel drives for brushless DC and AC servo motors the phase as well as the magnitude of the resultant output current vector must be accurately controlled. Many of the available AC servo motor drives use a three phase current loop topology. In such a design a separate feedback current loop joins an output current sensor to a summing junction just downstream of the digital to analog converter in each of the three current supplies of a single drive for a servo motor. To gang such drives in parallel requires a further level of sophistication not inherent in DC or AC drives for single servo motors.
The invention comprises implementation of drive designs for AC servo motors which allow several AC drives to be connected in parallel and allow for the connection and reconnection of AC servo motors to be accomplished primarily through a central processing unit (CPU) that obviates the need for memory units or DIP switches unique to each individual AC servo motor drive. Multiple AC servo motor drives are ganged under the supervision of the CPU.
In order to accomplish the necessary coordination of multiple AC servo motor drives paralleled to a single AC servo motor, the required motor current magnitude and phase (current vector) are broadcast by an axis controller to the drives. Thus, the several drives sharing the one motor load must all have simultaneous access to the command current vector. The command current vector for the drives is broadcast over a high speed serial data bus which allows many simultaneous listeners (AC servo motor drives in modular form and a CPU). To protect against damage to each AC servo motor drive, each of the three current output terminals is preceded by an output filter. The output filters allow the outputs from several drives to be wired in parallel (phase by phase) without the necessity of synchronizing the pulse width modulators (PWMs) among all the drives. Without the filters, large, potentially destructive currents could flow between the connected outputs from the drives.
Thus, each AC servo motor drive module provides power for each of the three phases of a Y-connected motor. (The new AC servo motor drive modules are also applicable to xcex94 connected motors.) Not only can the drive modules be ganged in parallel for increasing the amperage capacity and thereby the horsepower size of a servo motor, but additional modules can be added in parallel to provide redundancy in the event that an individual module fails. Thus, in sensitive installations the remaining drive modules will continue to fully power the servo motor.
In the preferred installation all of the AC servo motor drive modules, the CPU, the power supply, the input/output module and the axis controller for the command current vector are rack mounted units. The rack format allows all of the various functional modules to be plugged into a rack backplane with a bus structure built therein. The functional modules derive their power and commands from the bus structure. However, the new backplane configuration includes a motor drive power bus and serial data busses between the modules on the rack as opposed to parallel data busses.
Inclusion of the motor drive power bus on the rack backplane provides a machine controller that eliminates the task of wiring a motor power supply to each AC servo motor drive module. Moreover, the serial data busses require significantly fewer wires than parallel data busses. In the preferred embodiment the rack backplane supports six independent busses and minimizes the number of connections to each module and expansion rack. The AC servo motor drive modules can be combined in parallel merely by plugging in jumper wires to the appropriate sockets under the fronts of the modules.